Electrical musical instrument



L. HAMMOND ET AL ELECTRICAL MUSICAL INSTRUMENT Nov. 12, 1940.

Filed Nov. 17, 19.58

w; "a ade-14m ill Patented Nov. 12, 1940 UNITED STKTES PATENT OFFICE ELECTRICAL MUSICAL INSTRUMENT tion of Delaware Application November 1'1, 1938, Serial No. 240,889

1'7 Claims.

Our invention relates generally to electrical musical instruments, and more particularly to improved swell or intensity control circuits and apparatus for instruments of this type.

It is desirable in an electrical musical instrument that means be provided for varying the intensity or volume of the over-all acoustic output of the instrument by relatively small increments, it being preferable that the increments be of such small value that they are not perceptible to the ordinary listener. The apparatus should be of such construction that it is very durable and that it does not introduce transients in the output. An ordinary variable resistance or potentiometer in the otuput circuit is not fully satisfactory because of the rapid mechanical wear of the slider and the resistance, and because of the possibility of accumulation of dirt tending to impair the electrical contacts. While a variable resistance of this character is satisfactory for radio receivers and similar apparatus, the almost constant use of the swell or expression pedal by an organist renders this type of control unsatisfactory for organs. Similar difliculties appear in connection with the tremulant apparatus which may be considered as a periodically varying intensity or volume control device.

An improved apparatus for the swell or expression pedal of an instrument of this character is disclosed in the application of Laurens Hammond, Serial No. 91,284, filed July 18, 1936, wherein the difliculty of mechanical wear is substantially obviated, but in which the increments of impedance control by the swell pedal are relatively large if the number of switch contacts is to be reduced to a number which makes the apparatus reasonable in cost.

' These disadvantages of prior swell control apparatus for electrical musical instruments are overcome by our invention wherein the incremental changes in intensity of the output may be infinitely small, depending solely upon the operation of the swell pedal by the musician. The apparatus is also free from parts subjected to appreciable mechanical wear, and thus the apparatus will outlast the instrument as a whole. Furthermore, its operation does not introduce undesirable transients.

It is thus an object of our invention to provide an improved swell control apparatus for electrical musical instruments which is very durable, may be manufactured at a low cost, and which is capable of controlling the amplitude of the output signal by substantially infinitely small in- 5 Other obiectcs will appear from the following description, reierence being had to the accompanying drawing in which,

Figure l is a wiring diagram of representative portions of a complete electrical musical instrurnent showing particularly the swell or volume control circuit; and

Figure 2 is a diagrammatic view of a tremulant producing mechanism which may be incorporated with the swell control apparatus.

Referring to Figure l, the instrument includes a plurality of divider tubes BT13 and D'I'Bl which are representative of a complete set of tubes, one for each note of the musical scale, and which are arranged in stages in such manner that the frequency delivered by each tube is controlled by a preceding stage delivering a signal having twice the frequency of that supplied by the controlled stage. Thus, in the complete instrument there are l2 vacuum tube oscillators, or other similar sources of electrical impulses of the frequencies of the twelve highest notes of the musical scale, and each of these sources will supply a controlling signal to the highest of a plurality of cascaded stages for the corresponding octave notes of the instrument.

The tubes DTI3 and DTSI are representative of two such stages, the signal from a controlling source to the tube DT'l3 being impressed upon the grid thereof through a condenser Ca and resistance R3, the grid being connected to ground through a high resistance R4. The cathode of the tube DTl3 is connected to ground through a resistor R6 and a condenser Cc, and is connected to the grid by a condenser Cb. The plate of the tube BT13 is supplied with current from a suitable source indicated as +300 V., is con nected to ground through a condenser Cd, and supplies a signal to the grid circuit of the tube DTBI of the following stage through a condenser Ca and a resistance R3 in the same manner as the tube CT13 receives a signal from the controlling source. This system of generating the various frequencies of the musical scale is disclosed and claimed in greater detail in a patent to Laurens Hammond No. 2,126,682.

The signal used for the production of the note is derived from the tubes DTGI and DT13 through condensers Cf connected to the cathodes of these tubes. These signals are impressed re- 2 s,sa1,1as

spectively upon the grids of control tubu c'fil and CTIO, the wave form being changed as desired by resistors RIO and condensers C and C0 which are connected in parallel with the resistor g RIO between the grid of tube CTOI and C110 and ground. The condensers C0 are provided with gang switches by which they may be simultaneously disconnected from the circuit.

The signals from the divider tubes are thus continuously impressed upon the grids of the control tubes but the latter are normally biased so as to be non-conducting and are rendered conductive only upon operation of key controlled switches by which the cathode-platecurrent of 00 these tubes is controlled in a manner to deter- 'mine the intensity envelopes of the notes. This key controlled switching means comprises manually operable keys KOI and K10 which control the plate-cathode current, which is supplied I from a plurality of sources of adjustable direct current potential Ba. Bb and Be. The source Ba is normally connected to a condenser C0 through a resistance Hi0 and switch I02, while the source B0 is connected through a switch I00 and re- S sistance R0 to a terminal M which in turn is connected to the cathode of the tube CTN. through a resistance R1, the cathode being connected to ground through a condenser C0. A condenser Cl connects the terminal M to ground.

I The switch I02 is adapted upon depression of the keytomakecontactwithafixedswitch arm I00 whichisconnectedtotheterminalM through a resistance R0, whereupon the potentialofsourceBcisimpressedupontheterminal 00 M through a resistance RII. the resistance RII and the resistor R0. By this means, the intensity envelope of the note is controllable. as more fullydisclosedandclaimedinthemtentto Laurens Hammond No. 2,126,464.

0 The switch I00, when closed, permits the potentialofthepointMtorisesuiiicientlysothat the control tube will be cut oil, the rate at which thepotentialoftheterminalMrisesbeingdetcrminedinpartbytheresistanceR0. When itisdesiredtosustainthenotuasustaining pedal I00isoperatedtoswingarockshaft III clockwise and, through an insulating strip II2 carried by the shaft, depress the flexible arms of switches I00. Under these circumstances, the

'0 potential source Bb is disconnected from the terminal M, and the note will continue to sound after the key is released until the flow of current through the resistor R1 depletes the charge of the condenser 01.

u The plates of the control tube CTII and CT" are illustrated as connected to a common conductor IIO which leads to adjustable resonant output circuits illustrated by. a block III. A plurality of conductors Ill are provided, each connected to the plates of a plurality of the control tubes so that the adjustable resonant circuits may be individually controlled for groups of notes.

The composite signal from all of the control as tubes, as modified by the adjustable resonant output circuits. is connected to a preamplifier through a blocking condenser CIO, being connected to the grid III of a tube I20 in the first stage of the amplifier. The grid III is connected !0 through a grid resistor R to a source of biasing potential provided by the resistance network RII, RIO, R", the latter being connected to a biasing potential source indicated as being of -200 volts. The plate supply is indicated as 10 provided by a source marked B +250 volts, being con.nected to thepiate through a plate resistor RI Connected between the grid III and the plate I20 of the tube I20 is a network consisting of a blocking condenser CI2 and variable condensers Oil and 0" connected in series with the 'condenser CI2, the condenser CIO being shunted by a variable resistance R20. 'A resistor RIO is connected between the condenser CI2 and the junction point I20. The plate to grid circuit which 10 includes the condensers CIO, CIO and CIO forms an adjustable degenerative or inverse feedback. The variable condenser Cll is controlled by a swell pedal I20 through a suitable actuating linkage, and the condenser CIO and its shunting resistance R are provided to produce an adiustable tremulant eifect as will be pointed out here- The values of the elements of the circuit are such that the degenerative feedback may be varied from substantially zero feedback to substantially 100% feedback. Since the variable condenser CIO is positioned in this feedback circuit. it may be of relatively small value, and may thus be of the air dielectric type customarily u used for radio frequency tuning. The values of the condensers CI2, CIl, CIO are resistors RIO, R, are such that the degenerative feedback is substantially uniform for all frequencies within the range of the instrument, although if variable so frequency distortion is desired, this mesh may be modified to introduce such distortion.

The "cathode, screen and suppressor of the tube I20 are connected in the usual manner, and the plate I22 is connected to the grid I20 of a tube 00 I00 forming the next stage of the preamplifier through a blocking condenser CH. The output of the preamplifier (which may include additional stages) is connected to a power amplifier I02, which in turn supplies the amplified signal to the speaker I00.

The condenser CIO for providing the tremulant isshowninl igure2ascomprisingapairof semi-circular or segmental plates I00 and I01, the plate I00 being secured to an insulating block 5 I00 by a bracket I00, mounted in a housing I00. The plate I01 is carried on the end of a shaft I02 which is insulated from its driving shaft I by an insulating coupling I00. The shaft I is driven through a suitable speed reducing gear train from a motor I00, the supply of current to which is controlled by a rheostat I00 to permit adjustment of the speed of the motor. A brush I02 is mounted on an insulating block I00 within the housing I00. The brush I02 and the bracket I00 thus form the terminals of the condenser, and leads from these terminals are shunted by a variable resistor I00 operated by a control knob I00.

The plate I01 is preferably rotated at a speed go in the order of 7 c. p. s. The degree of feedback is thus periodically varied, and hence produces a periodic variation in the intensity of the sound produced by the loud speaker I00. By proper adjustment of the control knob I00, the degree of amplitude variation produced by the condenser CIO may be varied from substantially zero to a value which will cause a pronounced tremulant in the music produced by the instrument. 7

Since the condenser CI is in series with the condenser CI, its effect on the signal feedback will be greatest when the condenser Oil is adjusted to its maximum capacitance, (minimum reactance), and conversely, when the condenser Cl! is adjusted to minimum capacitance (maximum reactance) the effect of the periodic variations of the capacity of condenser Cl. upon the feedback will be least.

Thus, the periodic variations in the capacity of the condenser CIG will have relatively little effeet on the gain of the amplifier when the swell pedal is open (maximum sound intensity), and a relatively great effect when the swell is nearly closed (minimum sound intensity). of course the effectiveness of the variations in the capacity of condenser CIG will change correspondingly for intermediate positions of the swell pedal. This is a desirable characteristic since, from the point of view of the musician, a pronounced tremulant is much less desirable in music played forte or fortissimo than in music played piano or pianissimo.

By proper adjustment of the control knob I58, the degree of amplitude variation produced by the condenser Cl6 may be varied from substantially zero to a value which will cause a very pronounced tremulant in the music produced by the instrument.

The following are given as illustrations of the values of resistors and condensers which may be used in the first stage of the preamplifier, it being understood that these values may be varied considerably without materially afiecting the operation of the apparatus:

RM 5 megohms RIB 2000 ohms RIG 500() ohms RII 150,000 ohms RIB 350,000 ohms RlS 1 megohm R20 0-100,000 ohms CIO .0002 microfarad Cl2 .05 microfarad CI'I .02 microfarad CH variable 10 to 500 micro microfarads Cli variable 10 to 1000 micro micrafarad From the above description, it will be apparent that as the capacitance of the condenser CM is increased, the amplitude of the signal fed back from the plate I32 to the grid I [8 of the tube I20 will be increased, and since the plate signal is substantially 180 out of phase with respect to the signal supplied to the grid through the condenser Clll, the latter signal will be attenuated to a degree dependent upon the setting of the condenser CM. Similarly, when rendered effective by adjustment ofthe control knob I58, the rotating plate condenser CIS will cause periodic variation in the intensity of the feedback signal impressed on the grid H8 and thus produce the desired tremulant effect in the sound output of the instrument.

The feedback mesh is of such character and its elements of such value, that the frequency distortion introduced is negligible. In some instruments it may, however, be desirable to provide elements of such values that the bass notes are attenuated with respect to the treble notes as the overall intensity is increased. Since the capacities of the condensers CH and CIS are varied by infinitely small increments, their operation will not introduce undesirable transients.

The degenerative feedback circuit is illustrated as receiving its feedback signal from the first tube of the preamplifier. It will be apparent, however, that the feed back circuit might well be connected to the plate of the third tube of the preamplifier, or to any other point in the preamplifier circuit in which the signal is substantially 180 out of phase with respect to the signal supplied to the grid in the first stage of the preamplifier. similarly, the feedback circuit might be applied to a tube other than that of the first stage of the preamplifier. However, it has been found that the circuit arrangement illustrated is productive of very satifactory results, and that it has some advantages over the slightly modified circuit arrangements which might be used to accomplish substantially the same results.

We therefore intend, in the following claims, to include within the scope of our invention, all such equivalent circuits and arrangements whereby substantially the results of our invention may be obtained by substantially the same means.

We claim:

1. For use with an electrical musical instrument having a plurality of generators of electrical impulses of the frequencies of the musical scale, a signal output circuit, and selectively operable means to control transmission of impulses from said generators to said output circuit; an amplifier having a multi-electrode electron discharge device including a cathode, grid and plate, an input circuit and an output circuit for said device, means for impressing the signal from said signal output circuit upon the input circuit of said device, a swell pedal, a degenerative feedback circuit from the output circuit of said device to the input circuit thereof, said feedback circuit including a variable condenser operated by said swell pedal, and electroacoustic translating means connected to the output circuit of said device.

2. The combination set forth in claim 1, in which an additional variable condenser is connected in series with said swell pedal operated variable condenser, and means are provided selectively to vary the effective capacitance of said additional variable condenser at a periodic rate.

3. The combination set forth in claim 1, in which said degenerative feed-back circuit includes a fixed condenser connected to the plate of said device in series with said swell pedal controlled variable condenser, and in which there is a high resistance circuit connected between the respective plates of said condensers which are connected together and the cathode of said device.

i. The combination set forth in claim 1, in which said signal output circuit is connected to the input circuit of said device through a blocking condenser, and in which said degenerative feedback circuit is substantially free from undesirable frequency distortion throughout the musical frequency range.

5. In an electrical musical instrument having means for electrically generating and controlling a musical signal, an amplifier connected to said generating and controlling means to amplify said signal, said amplifier including an electron discharge device having a cathode, plate and grid, 9. point in said amplifier in which the signal is substantially 180 out of phase with respect to the signal supplied thereto, a degenerative feedback circuit from said point to the grid of said device, said circuit including a periodically variable condenser to cause periodic variation in the amplitude of the signal in the output of said device, thereby to produce a tremulant efiect.

6. The combination set forth in claim 5 in which said variable condenser comprises a pair of plates, and a motor is provided for moving one of said plates periodically to vary the efiective capacitance of said condenser and thereby to cause periodic variation in the gain of said device for the purpose of producing a tremulant eifect.

7. The combination set forth in claim 5 in which said variable condenser comprises a relatively fixed plate, a cyclically movable plate, and in which said variable condenser is shunted by a manually variable resistance thereby tovary the effectiveness of the periodic changes in capacity of said variable condenser to produce a tremulant effect. 1

8. The combination set forth in claim 5 in which said variable condenser is provided with a resistor shunt for manually adjusting the effectiveness of said capacitance, and in addition, is provided with power operated means for varying its effective capacitance at a tremulant periodicity.

9. In an electrical musical instrument, the combination of means for generating and controlling a signal translatable into musical sound, means for amplifying said signal including an electron discharge device having an input circuit connected to said means for generating and controlling said signal and anoutput circuit, electroacoustic translating means connected to said output circuit, and a variable degenerative feedback circuit between said output circuit and said input circuit and operable substantially uniformly to vary the gain of said device throughout the frequency range of the signals produced by said signal generating and controlling means.

10. The combination set forth in claim 9 in which said degenerative feedback circuit includes a periodically varying reactance elective periodically to vary the degree of feedback and thereby to produce a tremulant effect in the output circult.

11. The combination set forth in claim 9 in which said degenerative feedback circuit includes twovariable reactances, one of which is manually adjustable and the other of which is provided with means for periodically varying its reactance, and with means for adjusting its effectiveness, thereby to vary the effective gain of said amplifier and to produce an adjustable tremulant effect.

12. The combination set forth in claim 9, in which said degenerative feedback circuit includes a variable reactance effective to vary the degree of feedback, and a swell control device is provided for controlling said variable reactance.

13. A volume control device for electrical musical instruments in which the output is transmitted through an amplifying electron discharge device having a cathode, grid and plate, comprising a variable condenser, and an inverse feedback circuit connecting said condenser between the plate and the grid of said device.

14. In an electrical musical instrument having a signal output circuit and having means for amplifying and translating into sound electrical signal impulses, the combination of an electron discharge device having a cathode, grid and plate, a condenser connecting said grid to the signal output circuit of said instrument, a circuit connecting the plate of said device to said amplifying and translating means, a terminal at which the signal is substantially 180 out of phase with respect to the si nal on said grid. and means to 5 control the acoustic output volume of said translating means comprising. a circuit connecting said terminal to said grid, said circuit including a variable condenser, and an expression pedal associated with said instrument for operating said variable condenser.

15. In an electrical musical instrument having a signal output circuit and having means for amplifying and translating into sound electrical signal impulses, the combination of an electron discharge device having a cathode, grid and plate, a condenser connecting said grid to the signal output circuit of said instrument, a circuit connecting the plate of saidv device to said amplifying and translating means, and means to control the 20 acoustic output volume of said translating means comprising, a feed-back circuit connecting the plate of said device to the grid thereof, said feedback circuit including a variable condenser in series, an expression pedal associated with said 26 instrument for operating said variable condenser, and a high impedance circuit connecting the grid of said device to the cathode thereof.

16. In an electr. al musical instrument having a signal output circuit and having means for amplifying and translating into sound electrical signal impulses, the combination of an electron discharge device having a cathode, grid and plate, a condenser connecting said grid to the signal output circuit of said instrument, a circuit connecting the plate of said device to said amplifying and translating means, and means to control the acoustic output volume of said translating means comprising, a feed-back circuit connecting the plate of said device to the grid thereof, said feedback circuit including a variable condenser in series, optionally operable means for continuously varying said variable condenser at a tremulant rate, and a high impedance circuit connecting the grid and cathode of said device.

17. In an electrical musical instrument having a signal output circuit and having means for amplifying and translating into sound electrical signal impulses, the combination of an electron ducharge device having a cathode, grid and plate, a condenser connecting said grid to the signal output circuit of said instrument, a circuit connecting the plate of said device to said amplifying and translating means, and means to control the acoustic output volume of said translating means comprising, a feed-back circuit connecting the plate of said device to the grid thereof, said feedback circuit including a variable condenser in series, optionally operable means for continuously varying said variable condenser at a tremu- Q lant rate, an adjustable resistance shunting said variable condenser, and a high impedance circuit connecting the grid and cathode of said device. 

